This invention relates to a lapping tool utilized for lapping such articles as a silicon wafer, an optical lens, etc., and a method of manufacturing such a lapping tool.
Although the lapping tool of this invention can be used in many applications, the invention will be described hereunder by taking lapping of a silicon wafer as an example.
When lapping a silicon wafer, the wafer is interposed between rotating upper and lower lapping tools in the form of discs, and abrasive particles in the form of a slurry are supplied between the upper and lower surfaces of the wafer and the inner surfaces of the upper and lower lapping tools. As the lapping tools are rotated under pressure, thin surfaces of the wafer are cut away by the cutting force of the abrasive particles, thus finishing the surfaces of the wafer. In other words, the flatness of the lapping tools is transfer printed onto the surfaces of the wafer.
Such lapping is used to make flat or smooth the surface of a silicon wafer, a jewel, a metal article, optical lens, glass, ceramics, etc. and the demand for lapping is increasing with year as a result of rapid development of electronic technique.
Generally, in the lapping operation of a silicon wafer, fine particles of alumina (Al.sub.2 O.sub.3) or zirconia (ZrO.sub.2) are used as abrasive particles. The particle size of the abrasive particles is directly related to the lapping efficiency, the surface coarseness of the workpiece, and the depth of a surface layer of the workpiece which has residual stress and cracks by lapping. Usually, a particle size of from #1000 to #1200 is used. The lapping property of a silicon wafer is influenced by the affinity or lubrication between the lapping tools and the abrasive particles. When the base material or matrix of the lapping tool consists of steel, the affinity between the abrasive particles and the lapping tool is poor so that after lapping, a large number of scratches are formed on the surface of the workpiece. For this reason, in recent years, cast iron containing graphite is generally used. More particularly, the result of experiment shows that cast iron containing spherical that is spheroidal graphite is especially suitable for lapping a silicon wafer. Thus, the state of crystallization of graphite has a large influence upon the quality of the surface of the lapped silicon wafer, but the optimum condition of the state has not yet been clearly understood.
Although the flatness of the lapping tool has a direct influence upon that of a wafer and since the flatness of the lapping tool is transfer printed on the surface of the wafer, the flatness of the lapping tool is governed by the hardness and uniformity of the lapping tool. The prior art lapping tool made of spherical graphite cast iron utilizes a material having a bull's eye structure corresponding to JIS FCD 45 (JIS means Japanese Industrial Standard) or a material having a ferrite structure corrsponding to JIS FCD 40. Both materials have a hardness of about Hv 100-200 (measured with Vickers hardness measuring apparatus under a load of 500 g) so that they are soft and liable to be worn out. For this reason, these materials have a problem of degrading the flatness. Furthermore, the prior art cast iron lapping tool is not homogeneous, so that the lapping tool is lapped nonuniformly by the abrasive particles which presents another problem of degrading the flatness.
During the lapping operation, the lapping tool itself is abraded by the abrasive particles, so that fins are formed at the ends of the grooves formed on the lapping tool. These fins drop onto the surface of the wafer thereby damaging the surface thereof. The larger is the elongation characteristic of the matrix of the lapping tool, more significant is the formation of the fins. This also causes a problem for the ferrite structure of the materials corresponding to FCD 45 and FCD 40. Where the material comprising the lapping tool contains a precipitated phase of hard and coarse carbides of molybdenum, manganese and chromium, for example, or contaminants, those phase and contaminants, the fins would drop off as the lapping tool is worn out and damage the wafer surface.